Patent GB-940940-A (GB)
Title: Improvements in or relating to gauging material properties
Inventor: ALEXANDER FRANK MAXWELL
IPC: G01N23/203
Filed: 1961-06-28
Abstract: 940,940. Nuclear radiation backscatter gauges. INDUSTRIAL NUCLEONICS CORPORATION. June 28,1961, No. 23466/61. Heading G1A. A backscatter gauge for determiningthe weight per unit area or thickness of a moving sheet of material 14, Figs. 1 and 2, e.g. paper, by measuring the intensity of radiation, e.g. electrons from a radio-active source 12 reflected by the material back to a detector 10, e.g. an ionisation chamber, mounted with the source 12 at a predetermined distance D from the material is calibrated by withdrawing the source/detector assembly 10,12 from the material and inserting a standard reflector plate 30 between the assembly 10,12 and the material 14 so that in the calibration mode, Fig. 2, the assembly 10,12 is spaced from the reflector 30 by the predetermined distance D. The output from the detector 10 is coupled through a measuring circuit 26 to an indicating, recording and/or controlling instrument 28 and during the calibration mode the circuit 26 may be automatically standardised as described with respect to Figs. 2 and 6 of U.S.A. Specification 2, 951, 161 to compensate for contamination or decay of the radiation source and changes in the measuring circuit. As described the source and detector are moun- In a vertical cylindrical container 10, Fig. 6b, supported by vertical slides, Fig. 4 (not shown), in a screened housing having a horizontal base plate 40 parallel to the sheet under test, a circular aperture 242 in the base plate 40 being concentric with the container 10. The container 10 is movable vertically in its slides by means of a crank mechanism, Figs. 4 and 9 (not shown), actuated by a cam on a horizontal drive shaft which also actuates a rack and pinion to move the reflector plate 30 horizontally along slides on the upper surface of the base plate 40. The horizontal drive shaft is rotated by a pinion engaging a vertical rack, Fig. 9 (not shown), actuated by a fluid operated piston and cylinder and the cam producing the vertical movement of the source/detector container 10 is so shaped to provide a very rapid initial upward movement of the container 10 to allow the reflector plate 30 to move over the aperture 242, Fig. 6b, at the commencement of the calibration mode. The upper position of the container 10 during the calibration mode is determined by mechanical stops which are adjustable to ensure the correct spacing D between the container 10 and the reflector plate 30. The lower end of the container 10 is provided with a flanged metal ring 240, Fig. 6b, such that the flange 240 seats in the aperture 242 when the container 10 is lowered to the measuring position thereby preventing the entry of dirt into the mechanism and the source/detector may be protected by a thin window of organic sheet metal, e.g. "Mylar" (Trade name). When the aperture 242 is closed by the reflecting plate 30 in the calibration mode, the plate 30 acts as a shutter which prevents the escape of radiation from the housing and also prevents entry of dirt into the housing, the small gap between the base plate 40 and shutter 30 being sealed by felt strips 250, 252, Fig. 6b, over which the shutter 30 rides. U. S. A. Specification 2, 858, 450 also is referred to.